The invention is used to form an antenna that does not change the appearance of the radio device. In small-sized radio devices, such as mobile stations, such antennas usually have a planar structure: The antenna comprises a planar radiating element and a ground plane parallel with it. The electric properties of the planar antenna, such as the bandwidth and antenna gain, depend on the distance between said planes, among other things. When the mobile stations become smaller, even with regard to thickness, the distance mentioned above is inevitably reduced, whereby the electric properties deteriorate.
The internal space of a radio device can be used more efficiently by making the radiating element of the antenna such that it runs along the inner surface of the cover of the device. FIG. 1 shows an example of such an element known from the application FI 20012219. The conductive antenna element 110 is curved at its three edges so that the element has the same shape as the end part of the rear cover of the radio device. In the complete product, the element 110 is located against the rear cover. The element also comprises the antenna feed conductor FC and the short-circuit conductor SC that begin from its edge. The element 110 is an extruded piece in which a slot 117 has been machined before fastening so that the element is divided into two branches of different lengths, B1 and B2, as viewed from the short-circuit point. Therefore, the complete antenna is a dual band antenna.
Using a foil-like radiator located on the surface of the shell of the radio device is also known from before. FIG. 2 shows such a case. It is a simplified cross-section of a radio device equipped with an internal antenna, showing the cover 220 and main circuit board 201 of the radio device. A flexible antenna circuit board 210 of almost the width of the inner space of the radio device has been fastened to the inner surface of the cover by glueing. The radiating element 211 is a conductive foil belonging to the antenna circuit board and being located against the cover 220.
In the invention it is utilized thermoplastic material. Thermoplastic materials are suitable for moulding to a desired shape in a certain temperature. Their use in the industry is known as such. The use of thermoplastic material also in the manufacture of antennas is known from the application publication EP 0569016. It is in question a radar antenna, in which a number of components have been made by coating thermoplastic pieces with copper. After this, the components have been glued in place.